Measure the height, depth and width of the outside of the cabinets, using a tape measure. Cut two sheets of 1/2-inch-thick plywood to the depth and height of the cabinets. Cut a sheet of 1/2-inch-thick plywood to a length equal to the width of the cabinets minus 1 inch, and a width equal to the height of the cabinets.
Cut a sheet of 1/2-inch-thick plywood to the height of the cabinets and the depth of the cabinets minus 1/2 inch. Screw an L-bracket to the middle of one of the height-length edges of this sheet. Stand the sheet vertically above the cabinets, with the L-bracket against the wall. Mark on the wall where the screw hole in the bracket sits, using a pencil.
Remove the sheet. Drill a hole in the wall where you marked. Fill the hole with a wall plug. Stand the plywood sheet up again, with the hole in the L-bracket aligned with the hole in the wall. Screw the bracket to the wall using a 1 1/2-inch-long screw.
Screw the height and depth sized plywood sheets to the wall at either end of the cabinets, so the sheets are flush with the edges of the cabinet, in the same manner as you screwed in the middle sheet. Lift the height and width sheet above the cabinets, between the two end sheets. Screw the sheet to the end sheets and the middle sheet, using four countersunk 1-inch screws per sheet.